1. Field of the Invention
The present invention is related to a power supply voltage booster that boosts a power supply voltage to produce a voltage higher, and, more particularly, to a power supply voltage booster used in a fuel injection controller.
2. Description of the Related Art
A conventional voltage boosting circuit that charges a capacitor to a boosted voltage higher than a battery voltage is provided in a fuel injection controller. A fuel injection valve is operated by the controller based on, for example, a built-in solenoid that controls fuel injection in an internal combustion engine. At the start of the operation of a solenoid operated fuel injection valve, the controller executes an operation to supply a large current, such as the peak current induced by the boosted voltage, from the capacitor to an electromagnetic coil of the solenoid built in the fuel injection valve in order to immediately open the fuel injection valve. Then, the fuel injection controller executes an operation to provide a constant current supply such as a holding current for holding the valve open. The current is supplied from a constant current circuit to the electromagnetic coil in order to hold the open position of the fuel injection valve. To provide rapid opening response of the fuel injection valve, the battery voltage serving as the power supply voltage is boosted, thereby accumulating electrical energy in the capacitor. The accumulated energy is discharged from the capacitor so as to cause a large current to flow to the fuel injection valve, resulting in increased speed of the opening operation of the fuel injection valve.
The voltage boosting circuit used in such a fuel injection controller is of a DC-DC converter type equipped with a coil having one end to which a battery voltage is applied, and a switching device that is turned on and off for electrical connection and disconnection between the other end of the coil and the ground potential of zero volts serving as a reference electrical potential. The switching device is repeatedly turned on and off such that the capacitor is electrically charged from the counter-electromotive force of the coil when the switching device is turned off. A description of such a voltage boosting circuit is found in JP-2001-15332A.
FIG. 28A illustrates an example, of a voltage boosting circuit using an N-channel metal oxide semiconductor field effect transistor (MOSFET) as a switching device. In the voltage boosting circuit, a battery voltage VB is applied to one end of the coil L0. Two output terminals of the switching device T0—a drain and a source—are connected in series to the path between the other end of the coil L0 and the ground potential. The anode of a diode D0 is connected to the current path between the other end of the coil L0 and the drain of the switching device T0, which is closer to the coil L0. A charging capacitor C0 is connected in series to the path between the cathode of the diode D0 and the ground potential. In the present example, the ground potential is connected through a current detection resistor R0 to the source of the switching device T0, which is farther away from the coil L0, and the terminal of the capacitor C0, which is farther away from the diode D0.
As shown in FIG. 28B, when the switching device T0 is turned on in the voltage boosting circuit, the current is passed to the coil L0 through the switching device T0 and the resistor R0. When the switching device is switched from ON to OFF, the capacitor C0 is electrically charged through the diode D0 by a counter-electromotive force which occurs in the coil L0, the counter-electromotive force having a high voltage from several times to several tens times as high as the battery voltage VB. In this manner, the capacitor C0 is charged whenever the switching device T0 is repeatedly turned on and off. The diode D0 prevents a flow of current to the switching device T0 from the capacitor C0 that would result in a discharge of the capacitor C0.
In the voltage boosting circuit, a charging control circuit 10 turns on and drives the switching device T0 at the start of the switching control for boosting the voltage. Then, as shown, the charging control circuit 10 detects the current flowing through the switching device T0 and applied to the coil L0 as a result of the voltage produced across the resistor R0. Upon determining that the detected current increases to a predetermined switch-off threshold value Ioff, the charging control circuit 10 switches the switching device T0 from OFF to ON. The charging control circuit 10 detects the capacitor charging current flowing from the coil L0 into the capacitor C0 as a result of the voltage produced across the resistor R0 when the switching device T0 is turned off. Upon determining that the detected current is reduced to a predetermined switch-on threshold value Ion of approximately zero amperes in the present example, the charging control circuit 10 switches the switching device T0 from ON to OFF.
By repeating the above described operation of repeatedly switching device T0 on and off, the capacitor is gradually charged. The charging control circuit 10 monitors the charging voltage VC applied to the capacitor C0, which can be referred to hereinafter as the capacitor voltage VC. When the capacitor voltage VC reaches the target value of the target voltage for the capacitor charging, the charging control circuit 10 stops the switching control for boosting the voltage, and holds the switching device T0 off.
In connection with the above described action, certain observations have been noted. First, the above described type of voltage boosting circuit depends on a supply voltage for its step-up capability, that is, for the electrical energy needed for charging the capacitor per fixed time or unit time.
Specifically, the energy En stored in the coil L0 by one operation for switching can be expressed by Equation 1 as follows:En=½×L×Ioff2  (1)
where L is the inductance of the coil L0.
The time T required for executing one switching operation, that is, the time T required for one operation of turning on/off the switching device T0, can be expressed by Equation 2 as follows:T=L×Ioff/VB  (2)
Typically, the OFF time Toff of the switching device T0 is significantly shorter than the ON time Ton as shown in FIG. 28B, so that Toff is approximately equal to zero and T is approximately equal to Ton. Further, the time constant τ of the LR circuit constituted of the coil L0 and the resistance component when the switching device T0 is turned on is analogous to T=Ton.
It is seen from Equation 1 and Equation 2 that the step-up capability, that is, En/T, is expressed by Equation 3 as follows and is dependent on the battery voltage VB serving as the supply voltage.En/T=½×Ioff×VB  (3)
In the related art, as shown, for example, in FIG. 29, when the power supply voltage becomes lower, it is impossible to boost the capacitor voltage VC to a target value required for driving the fuel injection valve during the period between the time when electrical energy has been discharged from the capacitor C0 for fuel injection and the time when the next fuel injection is started. In short, the step-up capability is inadequate, possibly resulting in the inaccurate operation of the fuel injection valve. The same thing may possibly occur when the time interval between injections becomes shorter, that is, when the fuel injection valve is operated at shorter time intervals or when electrical energy is discharged from the capacitor C0 at shorter time intervals.